1. Field of the Invention
The present invention relates to a signal transmission apparatus, and more particularly to a signal transmission apparatus that is suitable for transmitting a delta-sigma modulated 1-bit signal.
2. Description of the Related Art
Recent portable electronic devices such as DVCs (digital video cameras) and DSCs (digital still cameras) have a variety of functions including, in addition to a moving image capturing function, a still image capturing function and an audio recording function, a function of playing back captured images and recorded sound, a function of editing captured images and recorded sound, a data transfer function, and so on. On the other hand, in order to achieve a high level of portability, a large number of electronic components are disposed in a very limited space of such an electronic device. Accordingly, they must be mounted in a very dense configuration employing a multilayer wiring board and fine patterns, and wiring patterns that connect the electronic components are disposed close to each other. Taking measures against noise resulting from crosstalk between such closely arranged wiring patterns is one of the major challenges when designing such portable electronic devices.
For example, an audio function may have a problem in that a transmission line that transmits a 1-bit digital audio signal output from a delta-sigma modulator and a digital circuit that drives the transmission line affect analog signals and clock signals. Delta-sigma modulators are often used for audio signal processing because they have a high level of dynamic range properties in the audio signal band and provide a 1-bit digital output, which can be easily transmitted.
On the other hand, the low-frequency component of the 1-bit digital audio signal that is output from the delta-sigma modulator is an analog audio signal. Accordingly, noise generated in the transmission line and the digital circuit that drives the transmission line, which is highly correlated with the analog audio signal, causes deterioration in sound quality when the noise is mixed with an analog input signal or sampling clock signal.
To address such a problem, Japanese Patent Laid-Open No. 9-186728 teaches that the reverse phase signal of a 1-bit digital audio signal is multiplexed by using a data rate that is double that of the 1-bit digital audio signal and transmitted, whereby the analog audio signal component included in the 1-bit digital audio signal is cancelled.
In addition, Japanese Patent Laid-Open No. 6-021824 teaches that a 1-bit digital audio signal is RTZ (return-to-zero) coded, to which the reverse phase signal is then added, so as to obtain equal numbers of logical ones for each digital state output to a transmission line, whereby the pattern of a transient current that occurs on the power source can be made independent of the 1-bit digital audio signal, and noise components from the transmission line and the digital circuit are cancelled.
However, with Japanese Patent Laid-Open No. 9-186728, the output signal synchronized with the leading edge of a sampling clock is still highly correlated with analog signals. Accordingly, there is a possibility that the sound quality might be deteriorated when noise synchronized with the clock is introduced into the sampling clock signal.
In addition, with Japanese Patent Laid-Open No. 6-021824, separate transmission lines, a positive phase signal transmission line and a reverse phase signal transmission line, are provided, and therefore the crosstalk between closely arranged wiring patterns is not completely cancelled.
Furthermore, with Japanese Patent Laid-Open Nos. 9-186728 and 6-021824, the number of transition points of digital output is double that of the output from the delta-sigma modulator. Accordingly, these conventional techniques also have a problem in that the power consumption of the digital circuit driving the transmission line is increased.